Implantable medical devices (IMDs), such as implantable pacemakers, utilize a variety of techniques and/or algorithms to detect heart arrhythmias. Ventricular arrhythmias, such as ventricular fibrillation (VF) and ventricular tachycardia (VT), are particularly dangerous, and can result in death if not quickly terminated by delivery of a therapy. Consequently, IMDs are programmed to deliver therapy, such as defibrillation or cardioversion shocks, upon detecting VF or VT.
IMDs typically detect VF and VT by measuring the intervals between ventricular depolarizations, i.e., ventricular cycle lengths (VCLs), and determining whether recently measured VCLs are indicative of VF or VT. However, supraventricular tachycardias (SVTs), such as sinus tachycardia, atrial fibrillation, atrial flutter, and reentrant atrial tachycardia, can be conducted to the ventricles, and can lead to short VCLs that falsely indicate VF or VT. Delivery of defibrillation or cardioversion shocks to the ventricles in situations where an SVT causes VF or VT to be falsely detected is generally not clinically needed and usually is ineffective in terminating the SVT. Moreover, defibrillation and cardioversion shocks, which can be delivered a number of times during an SVT episode that leads to false VF of VT detection, can cause significant patient discomfort or induce a VF or VT.
In order to avoid false detection of VF or VT during SVT episodes, some IMDs apply further analysis of the ventricular rhythm and additionally analyze the atrial rhythm to determine if an SVT is the cause of the fast ventricular rhythm, i.e., apply specific SVT detection rules. If the criteria for VF or VT is met during a particular ventricular interval and the further analysis indicates the presence of an SVT, an IMD typically avoids detection of VF or VT during that interval and, in some cases, delivers a therapy to one or more atria, such as anti-tachycardia pacing or a cardioversion shock. Nonetheless, because IMDs are programmed to err on the side of over-detecting potentially lethal VF and VT, IMDs occasionally inappropriately detect VF or VT when the fast ventricular rhythm is non-lethal, e.g., caused by an SVT, despite such additional analysis.
In order to avoid inappropriate delivery of defibrillation and cardioversion shocks, and the substantial patient discomfort associated therewith, clinicians in some cases program IMDs to prevent delivery of therapy for ventricular rhythms that are slower than a certain median of mean cycle length. This makeshift solution is not without problems, however. Slow arrhythmias can cause patient symptoms, such as fatigue, dizziness, and fainting, and can quickly accelerate into a more dangerous arrhythmia. Further, such clinician programming generally does not eliminate all inappropriate detection of VF and VT.